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Unfortunately, my house furnace is located in the attic. I leave in NH so in the winter time it gets pretty cold up here. Researching the web, I found some opinions that folks say "consider building a large insulated box around the entire furnace--leaving an opening at the bottom for combustion air intake. Doing these two things would dramatically reduce the amount of warm air getting into the attic from the furnace source".

I've heard this said when trying to put a 90% efficent furnace in an attic to prevent condensation freezing. But if the furnace doesn't draw air from a seperate combustion supply pipe, I don't see how boxing an 80% eff furnace in uncondition space would help. As soon as you allow cold air access to the furnace, you know the cold will migrate into the heat exchanger. Ductwork has more exposure in an attic. SO it must be well insulated.

I have applied for an energy star rating on our new house in NH. This was one of their recommendations:

Suggest that the HVAC air handler in the attic be enclosed in a well insulated equipment room (R-19 or greater) - and - ALL the ductwork in the attic should be insulated to R-19 or greater ( Not R-4.3 ). It is more costly to have your conditioned space heating/cooling air running through a poorly insulated HVAC system in the cold/hot attic.

I'm sure setups vary, but my furnace is fairly well insulated with foil faced insulation that seems to work well by the crude measure of feeling the outside of the box when the furnace is running. The air handler, on the other hand, is poorly insulated, with no insulation at all on the removable door in front.

Building a box sounds like a small box covering the furnace, a framed and insulated mechanical room sounds much more attractive. Just make sure to have adequit combustion air and room for the technician to operate.